In recent years, saving of power consumption at home and office has been gaining momentum along with an increase in power demand and concerns about global environment. With the growing consciousness of energy saving, people make efforts to turn off the power supplies of electrical devices frequently, to revise the preset temperature of air conditioning, and to do the like.
In order to know how much energy is actually saved as a result of these efforts, there is a method using power meters provided to respective households.
However, the power meter for each household is provided to a power switchboard installed at a stage before the distribution of power to the inside of the house, and is not capable of measuring power consumption of each electrical device at home or office.
Moreover, power is usually distributed to a plurality of electrical devices via a power strip connected to a wall outlet at home or office. The commercially available power strip, however, does not have a function to individually measure the power consumption of each electrical device.
The power consumption may be measured by a voltage measurement device electrically connected to the power line. This method, however, has a problem that, when a surge occurs in the power line, the voltage measurement device connected to the power line may be broken due to a flow of excessive current into the voltage measurement device. In addition, it is preferable to calculate the power consumption as accurately as possible.    PATENT DOCUMENT 1: Japanese Laid-open Patent Publication No. 04-6477    PATENT DOCUMENT 2: Japanese Laid-open Patent Publication No. 57-26761